The present invention relates to organic light emitting diode displays, and more particularly to increasing the light output from the emissive layers.
Organic light emitting diodes (OLED are a promising technology for flat-panel displays. The technology relies upon thin-film layers of materials coated upon a substrate. However, as is well known, much of the light output from the emissive elements in the OLED is absorbed within the device. Because the light emission from the OLED is Lambertian, light is emitted equally in all directions so that some of the light is emitted forward to a viewer, some is emitted to the back of the device and is either reflected forward to a viewer or absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film displays. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers, see Modification of polymer light emission by lateral microstructurexe2x80x9d by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and xe2x80x9cBragg scattering from periodically microstructured light emitting diodesxe2x80x9d by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled xe2x80x9cBrightness and Contrast Enhancement of Direct View Emissive Displaysxe2x80x9d by Chou et al. Mar. 2, 2001.
The use of micro-cavities and scattering techniques is also known, for example see xe2x80x9cSharply directed emission in organic electroluminescent diodes with an optical-microcavity structurexe2x80x9d by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches capture all, or nearly all, of the light produced.
It has been proposed to use a periodic, corrugated, grating structure to induce surface plasmon coupling for the light emitting layer in an organic luminescent device, thereby inhibiting lateral transmission and wave guiding of emitted light while increasing the efficiency and the light output of the structure. It is theoretically possible to couple up to 93% of the light emitted by the organic luminescent materials in an organic luminescent device. See xe2x80x9cExtraordinary transmission of organic photoluminescence through an otherwise opaque metal layer via surface plasmon cross couplingxe2x80x9d by Gifford et al., Applied Physics Letters, Vol. 80, No. 20, May 20, 2002. Gifford et al. disclose creating the grating geometry for photoluminescent surface plasmon coupling by exposing a photoresist on glass with an interferometric pattern, followed by depositing subsequent layers that replicate the underlying surface profile. This approach is not compatible with the current manufacturing techniques used to make active matrix OLED displays, since for top-emitting OLED display devices, a layer of thin-film transistors are formed on the substrate prior to forming the OLEDs. For bottom-emitting OLED displays, manufacturing starts with a glass substrate that is coated with a layer of conductive indium tin oxide (ITO) that is patterned to provide conductors for thin-film transistors located on the substrate. The use of photoresist to create plasmon inducing gratings is problematical because the photoresist is an electrical insulator, thereby isolating the underlying ITO conductors from the OLED materials. Gifford et al. also suggest that the use of surface plasmon coupling can be an efficient means for outcoupling electroluminescence in an OLED device by using shadow masks on any desirable substrate. The use of shadow masks is not practical to create the gratings because of the small dimensions of the gratings. They also disclose that they have fabricated an OLED that uses surface plasmon coupling on a silicon substrate, but silicon substrates are not conventional or practical for OLED display devices.
There is a need therefore for an improved organic light emitting diode display structure that avoids the problems noted above and improves the efficiency of the display for practical devices.
The need is met according to the present invention by providing a active matrix organic light emitting diode (OLED) display that includes a substrate; a thin-film transistor (TFT) layer formed on the substrate; a layer defining a periodic grating structure; a first electrode layer formed over the grating structure and conforming to the grating structure; an OLED material layer formed over the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic electrode layer(s).
In one embodiment, the OLED display is a top-emitting active matrix organic light emitting diode (OLED) display that includes a substrate; a thin-film transistor (TFT) layer formed on the substrate; an insulating layer formed over the TFT layer, the insulating layer defining a periodic grating structure, a first electrode layer formed over the insulating layer and conforming to the grating structure; an OLED material layer formed over the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic electrode layer(s).
In an alternative embodiment, the OLED display is a bottom-emitting active matrix organic light emitting diode (OLED) display that includes a substrate; a first electrode layer formed on the substrate, the first electrode layer having first portions defining a periodic grating structure and second portions free of such a grating structure; a thin-film transistor (TFT) layer formed on the second portions of the first electrode layer; an OLED material layer formed over the first portions of the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic layer(s).